High strength aluminum alloys and composites are required in certain applications, notably the aircraft industry where combinations of high strength, high stiffness and low density are particularly important. High strength is generally achieved in aluminum alloys by combinations of copper, zinc and magnesium. High stiffness is generally achieved by metal matrix composites such as those formed by the addition of silicon carbide particles or whiskers to an aluminum matrix. Recently Al-Li alloys containing 2.0 to 2.8% Li have been developed. These alloys possess a lower density and a higher elastic modulus than conventional non-lithium containing alloys.
The preparation and properties of aluminum based alloys containing lithium are widely disclosed, notably in J. Stone & Company, British patent No. 787,665 (Dec. 11, 1957); Ger. Offen. 2,305,248 (National Research Institute for Metals, Tokyo, Jan. 24, 1974); Raclot, U.S. Pat. No. 3,343,948 (Sep. 26, 1967); and Peel et al., British Patent No. 2,115,836 (Sep. 14, 1983).
Unfortunately, high strength aluminum-lithium alloys are usually characterized by low toughness, as evidenced by impact tests on notched specimens (e.g., Charpy tests, See: Metals Handbook, 9th Ed. Vol. 1, pages 689-691) and by fracture toughness tests on fatigue precracked specimens where critical stress intensity factors are determined.
There have been two basic techniques used to improve the toughness of Al-Li alloys.
1. Techniques commonly used for other aluminum alloys, such as alloying (Cu, Zn, Mg), stretching 1 to 5% before aging to refine precipitation, control of recrystallization and grain growth with Zr (0.1%) and the control of initial grain size by the use of powder metallurgy.
2. The production of dispersiods in amounts greater than needed for recrystallization control using 0.5 to 2% of Mn,Zr,Fe,Ti and Co to homogenize slip distribution.
In the last 10 years these methods have had some success but the toughness of Al-Li alloys still falls short of that of conventional aluminum alloys.
Conventional techniques, for improving the toughness of Al-Li alloys, have not included the use of a vacuum melting and refining treatment. Aluminum alloys are typically melted in air; although, vacuum melting is used by some manufacturers of high quality aluminum investment castings, such as Howmet Turbine Components Corporation who make castings of A357 and A201, to avoid dross formation. (Bouse, G. K. and Behrendt, M. R. "Advanced Casting Technology Conference", edited by Easwaren, published by ASM, 1987).
Howmet has also made experimental Al-Li-Cu-Mg investment castings by vacuum melting (Proceedings of the Al-Li Alloys Conference, held in Los Angeles March, 1987, pp. 453-465, published by ASM International) to reduce reactions between lithium and air and to reduce hydrogen pick up which occurs when lithium reacts with moisture in the air. Commercial Al-Li alloys are usually melted under an argon atmosphere which accomplishes these objectives less efficiently than vacuum but is an improvement over air melting.
Al-Li alloys although having many desirable properties for structural applications such as lower density, increased stiffness and slower fatigue crack growth rate compared to conventional aluminum alloys are generally found to have the drawback of lower toughness at equivalent strength levels.
Conventional high strength Al-Li alloys have resistance to stress-corrosion cracking in the short transverse (S-T) direction less than about 200 MPa (29 Ksi) in the peak aged to overaged condition, e.g., alloy 7075 has a threshold stress for stress corrosion cracking in the S-T direction in the range of about 300 MPa (42 Ksi) in the T73 condition to about 55 MPa (8 Ksi) in the T6 condition.
The magnesium-lithium family of alloys when manufactured by conventional techniques are known to suffer from stress corrosion cracking, overaging, instability and creep at low temperatures. Razim, et al., Advanced Materials & Processes, Vol.137, Issue 5, pp. 43-47 (May, 1990). Some of the problems in Mg-Li alloys have been associated with alkali metal impurities and it has been observed that Na levels above 20 ppm reduced room temperature ductility. Payne et al., JIM, Vol. 86, pp.351-352 (1957-58). Some Mg-Li alloy specifications set the Na limit to less than 20 ppm for Wrought products and 10 ppm for castings.